Vehicles equipped with diesel or another lean burn engine offer the benefit of increased fuel economy, however, control of nitrogen oxide (NOx) emissions from such engines is needed due to the high content of oxygen in the exhaust gas. In this regard, Selective Catalytic Reduction (SCR) catalysts, in which NOx is continuously removed through active injection of a reductant, such as urea, into the exhaust gas mixture entering the catalyst, are know to achieve high NOx conversion efficiency.
A typical lean burn exhaust gas aftertreatment system is described in U.S. Pat. No. 6,928,806 and includes a SCR catalyst for converting NOx in the engine exhaust gas mixture by means of injection of a reductant agent. NOx sensors upstream and downstream of the SCR are coupled in the path of the exhaust gas entering and exiting the SCR catalyst. The outputs of these sensors are read by controller 12 and may be used to determine the NOx conversion efficiency of the SCR.
The above described aftertreatment system enables detection of a malfunction in the NOx conversion system, however, the cause of the malfunction may be found in the urea-dosing system or in the SCR catalyst. For example, the urea-dosing system may be clogged, or the SCR catalyst may be deactivated.
Thus, there is a need of a diagnostic tool for diagnosis of SCR catalyst systems, to facilitate service and repair.
It is therefore desirable that the present invention provide a diagnosis method which is capable of accurately diagnosing the cause of malfunction.
The aspect of the present invention resides in a diagnostic method for testing malfunction of a Selective Catalytic Reduction (SCR) aftertreatment system for reduction of NOx in an exhaust gas passage of an internal combustion engine, the SCR system comprising a dosing system that injects a fluid reducing agent into the exhausts upstream a catalyst reactor and a downstream side NOx sensor for supervising the NOx emission in the exhausts downstream the SCR reactor, the diagnostic method comprising the steps of: setting the torque range and the speed range of the engine to a predetermined interval and adjusting the fluid agent mass flow to a level which is normal for this interval; measuring and registering a first mean NOx sensor value for the mass flow level, increasing the fluid agent mass flow to a level which is higher than normal for the interval; measuring and registering a second mean NOx sensor value related to the higher mass flow level; and comparing the two registered NOx sensor values and determining if on one hand, the first value is higher than the second value, or on the other hand, the first value is lower than the second value.
The other features of this invention will become understood from the following description with reference to the accompanying drawings.